1. Field of the Invention
Exemplary aspects of the present invention generally relate to a developing unit, a process cartridge including the developing unit, and an image forming apparatus including the process cartridge.
2. Description of the Background Art
Conventionally, image forming apparatuses equipped with a developing unit using a two-component developer including a toner and a magnetic carrier have been widely used. Among image forming apparatuses of this kind, there is known a type that maintains toner density of the developer in a certain range by supplying the toner from a toner cartridge to the developer in the developing unit from which the toner is consumed during development.
In such a configuration, it is likely that the carrier in the developer is barely consumed but repeatedly reused, thereby wearing out a coating layer of a surface of the carrier and/or attracting a toner resin and additives to the coating layer as an image is output. Consequently, charging characteristics of the carrier for charging the toner may deteriorate gradually. Namely, the carrier may be degraded.
When such degradation of the carrier progresses, electric charge of the toner is reduced, causing background contamination and/or toner dispersion. Thus, such an image forming apparatus requires periodic replacement of the carrier by a maintenance personnel, resulting in an increase in maintenance cost and unit price.
Japanese Patent Unexamined Application Publication No. 2005-292511 discloses a developing unit in which a premixed developer including a carrier and a toner is supplied to the developer in the developing unit so as to recover the toner density, while the amount by which the developer is increased is discharged outside the developing unit.
A new carrier in the premixed developer is supplied to the developer in the developing unit, while an old carrier is discharged from the developing unit little by little by discharging the developer. Accordingly, the carrier in the developer is replaced with the new carrier, thereby making it possible to replace the carrier easily.
In addition, the developer according to Japanese Patent Unexamined Application Publication No. 2005-292511 includes a developer discharge vent at a position where the level of the developer rises and falls depending on the amount of the developer added and reduced in a developer conveyance path.
In such related art developing unit, when the premixed developer is supplied, thereby increasing the total amount of the developer in the developing unit, the level of the developer in a developer supply conveyance path rises. When the level of the developer reaches the height of the developer discharge vent, the developer is discharged outside the developing unit from the developer discharge vent.
Japanese Patent No. 3264765 discloses a developing unit including a shutter serving as a developer discharging mechanism that opens and closes the developer discharge vent. When the toner density decreases and thus the premixed developer is supplied, accordingly the shutter is simultaneously opened to allow the developer to be discharged from the developer discharge vent.
In this developing unit, based on the toner density, the size of the opening of the developer discharge vent is regulated by the shutter such that a predetermined amount of the developer is discharged.
However, in the related art developing unit disclosed in Japanese Patent Unexamined Application Publication 2005-292511, the traveling speed of the developer being transported in the developer conveyance path and/or the torque of a developer conveyance screw serving as a developer transporter, which exerts a conveyance force on the developer, may cause the developer to leap, causing the excess developer to be discharged inadvertently from the developer discharge vent even when the amount of the developer in the developing unit is not increased.
When the developing unit has such a structure allowing the excess developer to be discharged outside even when the amount of the developer transported in the developer conveyance path is appropriate and/or the amount of the developer falls below the appropriate amount, the appropriate amount of the developer in the developing unit cannot be reliably maintained. In other words, despite the fact that the amount of the developer in the developing unit is not increased, there is a possibility that the developer may still be discharged inadvertently.
When the developer is discharged from the developer discharge vent regardless of the fact that the amount of the developer is less than the appropriate amount, the amount of the developer in the developing unit falls below the necessary amount of the developer in the developing unit. As a result, the developer is inadequately supplied to the developer bearing member, and thus a desirable image density may not be obtained.
By contrast, according to the related art developing unit having the shutter disclosed in Japanese Patent No. 3264765, the shutter is opened only when the premixed developer is supplied, allowing the developer to be discharged from the developer discharge vent. Accordingly, it is made possible to suppress inadvertent discharge of the excess developer caused by the torque of the developer conveyance screw or the like, so that the developer in the developing unit is maintained at an appropriate level.
However, according to this configuration, the amount of the developer to be discharged is regulated based on the toner density. In other words, the amount of the developer to be discharged is not regulated based on the amount of the developer in the developing unit. Thus, there is a possibility that when fluidity of the developer is reduced due to degradation of the developer and/or fluctuation in ambient conditions, it is difficult to smoothly discharge the developer from the developer conveyance path to the developer discharge vent.
Consequently, when the shutter is opened to discharge the developer, there is a possibility that a predetermined amount of the developer is not discharged from the developer discharge vent, thereby increasing the amount of the developer in the developer conveyance path more than necessary. As a result, there is a problem in that the developer is accumulated and overflows from the developing unit.